The Sydney Metro has reached a major milestone, with all tunnelling now finished on the Sydney metro City & Southwest project.

The final boring machine, named Kathleen after Kathleen Butler, the technical advisor to Sydney Harbour Bridge builder John Bradfield, finished the tunnelling part of the project when she arrived at the northern terminus of the second tunnel from Barangaroo to Blue Point.

Each tunnel was 885 metres in length; however, the second tunnel was built a month faster than the first, due to the incorporation of lessons from the first tunnel.

These changes included modifying the cutter head and changing tunnelling processes to better dig through the clay material below Sydney harbour.

Kathleen churned through 175,000 tonnes of sandstone, clay, and marine sediment to complete the dual harbour tunnels. At their deepest, the tunnels are 40 metres below sea level.

All in all, the five tunnel boring machines have dug through 31 kilometres of tunnels during the last 17 months.

Earlier this week the last concrete segments which line the walls of the tunnel were installed.

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